This invention relates to a method of producing a thin film magnetoresistive sensor. Hitherto, aluminum or an aluminum alloy has been used as a material for electrodes or wiring metals having a minimum line width of 0.5 to 2 .mu.m to be deposited on-to a magnetoresistive film, such as a permalloy (a Ni-Fe alloy) film, of the thin film magnetoresistive sensor. However, when the permalloy film and aluminum are heated to a temperature of 200.degree. to 300.degree. C. or higher, diffusion of atoms takes place therebetween. Therefore, during the vapor deposition of aluminum electrodes or in a subsequent heating step, the magnetic properties of the permalloy film are considerably degraded. The permalloy film is usually very thin, that is, about 200 to 400 A in thickness, and it is, therefore, very sensitive to the reaction with the electrode material. The reaction between the permalloy film and aluminum can be prevented by employing a multilayered electrode structure comprising a double layer structure of Mo and Al (or an Al alloy) which (hereinafter referred to as Mo/Al) is characterized by a barrier layer of Mo provided between the Al layer and the permalloy film to interrupt the reaction between Al and permalloy. This prevention of the reaction is due to the fact that Mo has a very high melting point and the diffusion of atoms will hardly occur at a low temperature. Furthermore, no reaction between permalloy and Mo occurs, at a temperature of not higher than 400.degree. to 500.degree. C. Accordingly, the thin film magnetoresistive sensor using a Mo/Al multilayered electrode can endure heat treatment at a temperature of 400.degree. to 500.degree. C., even after the formation of the electrodes. However, with the Mo/Al electrode, a reaction does occur between Mo and Al to form a very thin layer of an intermetallic compound such as Mo.sub.3 Al (hereinafter referred to as a Mo/Al reacted layer) at the interface between Mo and Al. Although the intermetallic compound layer is too thin to damage the electrical properties of the Mo/Al electrode, it has such a high chemical resistance that extreme difficulty is involved in making the electrode with a chemical etching solution.
When an Al monolayer film is used as an electrode, the fine processing of the Al film with a minimum line width of 0.5 to 2 .mu.m can be accomplished with an alkaline etching solution such as an NaOH or KOH solution, without attacking the permalloy film. Also, when Mo as a material for a monolayer electrode is provided on the permalloy film, a chemical etching solution similar to the one used for Al may be used. However, a multilayer wiring of Mo with a good electric conductor such as Al and Au is required, since the use of Mo alone leads to a higher wiring resistance. On the other hand, when the Mo/Al double-layer electrode is adopted, the formation of the above-mentioned intermetallic compound layer at the interface between Mo and Al results in that only the unreacted Al layer undergoes chemical corrosion, leaving the Mo/Al reacted layer and the Mo layer therebeneath intact. Therefore, an electrode having the Mo/Al double-layer structure is very difficult to make.
The above-mentioned intermetallic compound can be processed by using a chemical etching solution based on a strong acid such as aqua regia. However, such an acid etching solution will also attack the permalloy film and, accordingly, it cannot be used. Attempts have also been made to etch the intermetallic compound by using a variety of alkali solutions, with great difficulties.
The following references are cited to show the state of the art: (i) U.S. Pat. No. 4,052,748 and (ii) IBM Technical Disclosure Bulletin, vol. 17, No. 4 (September, 1974), pages 967 and 968.